1EQEMD 



FOX HILL DRAUBRIDGE 



BOSTON (NOS TIDE GAGE) 



12 16 20 ZA 28 32 

 TIME (HOURS) 



TIDE DATA - FOX HILL DRAUBRIDGE 

 JULY 29-30, 1984 



Figure 12. Tidal calibration data, Fox Hill Drawbridge 



closely match the tide gage data for the 29-hour period specified above. 



These adjustments are explained below. 



a. Cross-sectional areas and frictional characteristics of both 

 channels and the bridge openings were adjusted. The minimum 

 cell size of 500 ft was much larger than the channel width at 

 many of the constrictions. A smaller cell size would have 

 greatly increased modeling costs; therefore, using the 500-ft 

 cell size necessitated that the flow be adjusted through these 

 oversized areas by alterations in depth and friction. It would 

 have been convenient if the depth could have been adjusted on 

 the oversized cells so that cross-sectional areas would match 

 between model and prototype. However, due to the large tidal 

 range in the study area, matching cross-sectional areas would 

 have caused the channels to dry up well above low water. 

 Consequently, it was necessary to make these cells deeper than 

 the area represented in the prototype would justify. Higher 

 water levels would cause excessive flow through these oversized 

 channels. A compromise depth was selected so that the channel 

 would remain flowing at low water levels. At low water the 

 opposite problem would occur. Since the depth of the channels 

 in the study area is much greater than the compromise depth 

 used in the model cells, the flow restriction is higher in the 

 model than in the prototype. This causes a reduction in flow 

 in the model at low water. Therefore, in addition to the 

 compromise depth, frictional characteristics were made depend- 

 ent upon depth to produce smaller Manning's n values at low 



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